Human being has invented various devices to use wind energy to generate electricity, and such devices can be divided into two categories based on the deployment of the rotating shaft: horizontal axis wind turbines which have horizontally deployed main shaft, and vertical axis wind turbines (VAWTs) which have vertically deployed main shaft.
Current VAWTs utilize blades of certain airfoil profile. As shown in FIG. 1, the blades 1 are usually connected to the vertical main shaft 2 via the radial arms 3, and the wind rotor is fixed on the top of a tower 4, and is able to rotate around the its center (i.e., the main shaft). As the wind rotor diameter increases, the length of the main shaft becomes longer, and the diameter of the main shaft becomes bigger, thus making manufacturing thereof difficult and costly. What's more, wind rotors with increased diameter also increases requirements on the height and strength of the tower. Although Chinese patent CN200810108995.8 discloses a VAWT with a hollow main shaft, or a truss structure main shaft, when the wind rotor diameter increases to a certain degree, the main shaft diameter will become too big and difficult to be manufactured, which hinders the upsizing of VAWTs—VAWTs of 300 kW and 500 kW may be the limit, letting alone much bigger VAWTs.
Further, for the current VAWTs with fixed blade setting angle (rotation angle), when the wind rotor rotates, the magnitude and direction of the torque on the blade are changing all the time based on the blade's position on the wind rotor rotation orbit. At certain positions, the torque is bigger and at other positions smaller; at certain positions, the torque is positive and at other positions negative. For large VAWTs, the diameter of wind rotor is increased, and the rotation speed of the wind rotor is lowered, the torque changes on the blade become more significant. Therefore, the wind rotor of the VAWT usually has lower aerodynamic efficiency as the ultimate torque output of the wind rotor is the resultant torque on all the blades. To improve the efficiency, the blade rotation angle must be adjusted in real time based on the blade's position on the wind rotor rotation orbit.
Blade rotation angle adjustment is usually done by providing a pivot at the center of blade ends, thus the blade is able to rotate and therefore adjust the blade rotation angle. However, the driving torque needed is closely related to the blade size, position of the pivot, wind direction and speed, and blade angle, etc. Therefore, in actual application, the blade angle shall be fixed to ensure stable output of the VAWT based on the specific conditions (e.g., under certain wind speed and direction).
Chinese Patent CN200610028267.7 discloses that a desirable wind rotor radius shall be 1.8 times to 4 times of the blade width (chord length). Based on the disclosure, for a wind rotor of 12 meter diameter, a suitable blade width is approximately 2 meters; for a wind rotor of 30 meter diameter, a suitable blade width is approximately 5 meters; for a wind rotor of 40 meter diameter, a suitable blade width is approximately 7 meters; and for a wind rotor of 50 meter diameter, a suitable blade width exceeds 8 meters. For a VAWT with a 50 meter diameter and a 50 meter length wind rotor, its output at 13 m/s is around 1 MW. However, such a wide structure (50 meter high and 50 meter wide) is difficult to be manufactured and transported, thus, hinders the commercialization of the turbine.
Further, current wind turbines are usually designed and manufactured to meet the planned capacity. The installation site and rated output are so limiting that the installation site and wind energy can not be used fully and efficiently. For example, wind turbines are typically manufactured with a rated capacity of 10 KW, 30 KW, 50 KW, or 100 KW independently. Manufacturing and assembly of parts for wind turbines of various capacities are different, so are the manufacturing requirements and techniques to be used. Therefore, they are hardly suitable for mass production.